docs: def normalize(self, decl_string, arg_separator=None):
          """implementation details"""
          if not self.has_pattern(decl_string):
              return decl_string
          name, args = self.split(decl_string)
          for i, arg in enumerate(args):
              args[i] = self.normalize(arg)
          return self.join(name, args, arg_separator)